Electrical system



April 22, 1924. 1,491,402

H. s. HOLBROOK ELECTRICAL SYSTEM Filed June 17. 1921 v Inventor:

Henry Stahlay H0|brq0ka I byfiw/r 7 His Attorney Patented Apr. 22, 1924.

UNITED STATES ,PATEINT' OFFICE.

HENRY S. EOLBROOK, OI RUGBY, ENGLAND, ASSIGNOB TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

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. Application filed June 17, I921. Serial No. 478,406.

To all whom it may concern:

Be it known that I, Hmwar S'mmmr Hon- BROOK, a sub'ect of the King of Great Britain, residing at Rugb in the county 6 of Warwickshire, En lan have invented certain new and 'use ul Im rovements in Electrical Systems, of which t following is a specification.

My invention relates to electrical systems, 10 and articularly to those having transformers t erein, and has for its general object an improved arrangement of parts for protecting the transformers against internal faults. A In a type of protective system commonly used to this enda circulating current is em ployed; there being a current transformer on the' primary side ofthe main transformer and a current transformer on the secondary side, The secondary windings of these two current transformers are connected in series and are traversed by the circulating-current. Their ratios of transformation are proportioned to that of the main transformer so that they give the same secondary current on the assumption that the main transformer has no losses. A trip coil'or relay is connected in shunt across the two secondaries and no current passes through this tripping circuit so' long as the ratio of the current input and current output of the main transformerremains constant. This ratio is of Course upset if an internal fault occurs in the main transformer and a difl'erentialcur-flrent corresponding to the fault current will flow through the relay or trip coils and'disconnect the main transformer from the circuit. a.

As hitherto arranged a very sitive limit has existed with regard to t e minimum value of the fault current required to operate the tripping mechanism. The current input to the mam transformer must always exceed the current out ut multiplied by t e turn ratio of the win ings, by the exciting current required to produce the working flux in the main transformer core, and required also to provide for losses of the main transformer. In conse uenoe a differential current always flows in t e tripping circuit so that it is necessary to biasthe tripgling degice' against operation. Obviously, t e discrepancy between. input and output current 1s not constant but varies with themagnitude and power factor of the load. The necessity for biasing the trippin device cannot, therefore, be removed by atfjustment of the ratio or balance of the current transformer.

A further limit has also e'itisted' in the system above referred to. by reason of the kick or rush of magnetizing current which 1 occurs when voltage is switched on to a modern ower transformer, the magnitude of this kick depending on the ma etic condition of the core when thetrans ormer was switched out of circuit. As this rush of current flows through one current transformer only, it produces a current in the trippingcircuit.

One method at present employed for preventing the o ration of the automatic circuit breaker y reason of the two effects, above referred to, is to shunt the tripping circuit with a kicking fuse. This, however, has the great' disadvantage of increasingthe fault setting required b the relay.

y my invention the need or a kicking fuse is entirely doneaway with and also with the corresponding need for increased mechanical bias, since I make use of electrical means for neutralizing the no-load differential current that would. normally flow in the tripping device. This I accomplish by connecting a reactor in parallel with the primary of the main transformer with suitable connections for the primary current transformer.

. .For a mcrecomplete understanding of the nature and objects of my'invention, reference should be had to the following detailed description when taken in connection with the accompanying drawing, in which:

The fi'gure is a symbolic representation of an arrangement of parts forrotecting a transformer in accordance witlEmy invention.

Referring now to the drawing, 1 denotes the supply mains across which is connected the pr mary 2 of a main power transformer whose secondary 3 su plies current through 100 the mains 4 to any oad. In the primary circuit is the current transformer 5, and in the secondary circuit the current transformer 6, the secondary windings of these a current transformers5 and 6 being connect- 1 ed in series in the manner commonly employed for circulating current protection. Across the circuit including these secondariesis shuntedthe tripping coil or rela 7 which actuates the automatic circuit breaker In the practice of the present invention it-is preferable (on account of the voltam-- pere characteristic of transformer steel not being a straight line) to design the reactor to operate at the same flux density as the power transformer. It is also desirable to build the reactor core with the same quality steel as used in the core of the main transformer, and for the magnetic circuit of the 'reactor to be proportioned to thatof the maintransformer in such a manner that the current taken by' the reactor bears a constant ratio to the current taken by the power transformer, whatever the value of the voltage applied to the circuit, and whether it be applied gradually or suddenly.

In operation it is seen that the reactor 10 connected as shown substantially neutralizes the no-load differential current. which would otherwise flow to the tripping device at 7. The arrows, which denote the direction of current in the secondaries of the transformers 5 and'6 at the instant of rwkoning, indicate this current as traversing these secondaries in series. Neglecting for the moment the no-load primary current in the main transformer, the currents in the secondaries of transformers 5 and 6' would b'eequal, assuming that their ratios of transformation are properly proportioned; consequently this circulating current would consume all the secondary E. M. F.s of these transformers so that there would be under these circumstances no resultant difference tending to cause current to flow in the circuit of the tripping device.

As pointed out above, however, the noload primary current of the main transformer does induce normally a resultant difference in these secondary E. M. F.s which will cause current to flow in the circuit of the trippin device at 7 against which the device must fie biased unless compensated for in some manner.

The reactor 10, which is in series with the secondary of transformer 5 and that at 9 of the potential transformer, effects this compensation. As a consequence of these connections, when the reactor is proportioned as above indicated, current will flow in series through the secondaries at 5 and 9 and through the reactor which will consume the resultant differential E. -M. F. at 5 so that there is substantially no tendency for current to flow in the circuit to the tripping device at 7 under normal conditions or when no fault occurs.

The need for kicking fuses is thus entirely dispensed with in this arrangement, while the limit of sensitiveness of the. s stem becomes the characteristic of the re ay or trippingecoil at 7.

As will readil understood this protective system is equally as applicable'to polyphase transformers as to single phase trans formers. Having now described an embodiment of my invention which is at present the best means known to me for carrying the same into effect, I would have it understood that this is merely illustrative and that I do not mean to be limited thereby to the precise details shown nor restricted in the choice of recognized-equivalents exce t as defined in m claims hereunto annexe W 1st I claim as new and desire to secure by Letters Patent of the United States, is,-

1. In electrical systems for protecting transformers and th like, the combination with current transformers havin their primaries connected respectively in t e primary and secondary circults of the transformer to be protected and their secondaries connected in series, of a tripping device shunted across the secondary circuit of said current transformers, and electrical means connected across said secondary circuit for substantially neutralizing the normal difierential current tending to flow in the circuit of said tripping device.

' 2. In electrical systems for protecting transformers and the like, the combination with current transformers having their primaries connected respectively in the primary and secondary circuits of the transformer to be protected and their secondaries connected in series, of a tripping device shunted across the secondary circuitof said currenttransformers, and a reactor connected across said secondary circuit and proportioned so that the current traversing it substantially compensates for that normally tending to flow in the circuit of said tripping device.

3. In electrical systems for protecting transformers and the like, the combination with current transformers having their primaries connected respectively-in the primary and secondary circuits of the transformer to be protected and their secondaries connected in series, of a tripping device shunted across the secondary circuit of said current transformers, a potential transformer, and a reactor connected in series with its secondary, the circuit including said reactor being connected across the secondary circuit of said current transformers and having its constants proportioned substantially to compensate for the differential current which normally at no-load tends to flow in the circuit of the tripping device.

4. In electrical systems for protecting transformers and the like, the combination with current transformers having their primaries (connected respectivel in the primary andsecon'dar circuits 0 the transformer to be protecte and their secondaries connected in series, of an electrores onsive protective device shunted across t e secondary circuit of said current transformers, and a reactor in parallel relation with the primary of the transformer to be rotected across the seconda circuit of sai current transformers where y substantially to compensate for the difierential current tending to flow in the circuit of the electrores nsive device because of a discrepancy in t e normal input and output currents of the transformer to be protected and also because of the rush of ma etizing current when the transformer to E: protected is connected to 1y circuit.

a sup 5. n electrical systems for protecting transformers and the like, the combination with current transformers havin their primaries connected respectively in t is primary and secondary circuits of the transformer to be protected and their secondaries connected in series, of an electroresponsive rotective device shunted across the secon ary circuit of said current transformers, a potential transformer having its primary connected in parallel with the primary of the transformer to be protected, and a reactor in circuit with the secondary of said potential transformer across the secondary circuit of said current transformers, whereby substantially to compensate for the differential current tending to flow in the circuit of the electrores 'onsiv'e device because of a discrepancy etween the normal input and outut currents of the transformer and also because of the rush of magnetizing current when the transformer to be protected is connected to a supply circuit.

In witness whereof, I have hereunto set my hand this nineteenth day of May, 1921. H. S. OLBROOK. frr

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D. Warm. 

